Yes, you are right-- social recommendation is a very broad term. In fact, one of the first collaborative filtering systems, which looked at people with similar music tastes, was named "social" [Ringo95] by its creators at MIT. For our purposes, let us consider social recommendation as any recommendation which is based on preferences and actions of others ...

Recommendation research got a boost with the Netflix challenge, which means there are lots of quality papers on how to predict a 1-5 rating for items from a dataset of previous ratings. That is one field that seems to be saturated, but there are lots of open problems:

Cross-Domain Recommendation: Current systems are really good at learning preferences in...